近幾年來，發展地球低軌衛星進行各項地球科學研究任務成為一項趨勢，例如CHAMP衛星、與中華三號衛星（ROCSAT-3）等，前者已於2000年成功發射升空，ROCSAT-3亦預計於2005年發射並執行其科學研究。低軌衛星的科學任務包括提昇地球重力場模式精度、探測大氣濕度與壓力變化、氣象預報與地磁時空變化等研究，為了讓上述各科學研究能順利進行，低軌衛星之軌道資訊是必要且重要的研究課題之一。

本研究利用CHAMP衛星上裝載之GPS接收器所接收之雙頻觀測量與地面上40個IGS追蹤站組成三次差分無電離層觀測量，並改正觀測量中之對流層誤差、固體潮誤差、質量中心改正、地球自轉、天線相位中心偏差等誤差量，以純幾何之動態法求定低軌衛星之短弧軌道位置。研究結果顯示短弧軌道在radial、along-track、cross-track三分量上的平均RMS值為20、20與15公分，平均3D RMS為32公分；重複軌道之三軸分量平均RMS值分別為25、17與13公分，平均3D RMS為32公分。

In recently years, there is a tendency to develop low-earth orbit（LEO）satellites, as the CAHMP lunched at 2000 and ROCSAT-3 will lunch at 2005, to process many earth sciences researches. The science projects of the LEO include earth gravity recovery, earth atmosphere temperature and pressure retrieval, weather prediction and space/time variability of the magnetic field of the earth. Determine the LEO position is one of the important and necessary task in order to carry out those researches above.

This research process the ionosphere-free triple-differenced carrier phase observation which consist of double frequency GPS observations from the GPS receiver carried on the CHAMP and 40 IGS tracking stations. Besides, the tropospheric correction model, solid earth tide correction, mass center correction of the satellite, earth rotation correction and antenna phase center correction are incorporated. Finally, determine the satellite short-arc orbit with the pure geometry, kinematic, method. The results show that the average RMS of the short-arc orbit at radial、along-track、cross-track components are 20、20 and 15 cm, and the average 3D RMS is 32 cm. The average RMS of the overlap orbit at three components are 25、17 and 13 cm and the average 3D RMS is 32 cm.

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